That would be an honor.

I would imagine that it could be considered an honor to get a letter of a personal nature from the British Prime Minister. Considering that he is a very busy man, a personal note means that he took time out of his day to consider your issue and respond to you.

Honor if really from minister

If you get a real letter from the British prime minister it should be an honor. This means they took the time to write you a personal letter. Now if the letter is a form letter and clearly not personalized from the prime minister then it should be viewed as neither an honor nor in insult.

Letters from High Up Usually a Good Thing

If someone takes the time to personally write you from a high position in government, I seriously doubt that letter is meant to cajole or criticize anyone. Politicians rarely have time to pen letters to ordinary citizens and I would be flattered if someone of a high station wrote a letter to me, even if it meant bad news. At least I would know the letter isn't insulting.

I Would Say An Honor

I believe it would be an honor to receive a personal letter from the British Prime Minister. I would have to imagine there would have to be a fairly good reason for receiving such a letter and I doubt it would be anything bad. I can't imagine an instance where it would be an insult.

It is an honor to get a personal letter from the British prime minister

It is a huge honor to get a personal letter from the British prime minister. The British prime minister is a person of immense power. To get a personal letter from the British prime minister is a definite honor because to have a person of such power recognize you is not a small deal.